A typical automobile contains some 225 stamped sheet metal panels. Of these about 15 are outer panels while the remaining 210--or about 95% of the total--are inner panels. The outer panels are characterized by their smooth, free form or sculptured shape. As such, they are well represented mathematically in computer aided design (CAD) systems by parametric, piecewise polynomial, patched surfaces (for example, Gordon surfaces, Bezier surfaces, NURB surfaces) capable of capturing their aesthetic free form geometry. Most inner panels, on the other hand, are characterized by their irregular, multi-featured shapes, often consisting of pockets, channels, ribs, etc. designed as modifications to an existing base surface, such as an offset of an outer panel, and frequently representable by analytic expressions (e.g., planes, cylinders, etc.).
The prior art computer aided inner panel design methods do not recognize these differences. Inner panels are designed and represented using the same patch-based mathematical methods as are used to design and represent outer panels. This situation has evolved because of the lack of special purpose surfacing techniques. The force fit of outer panel techniques to inner panel design has lead to a number of problems and inefficiencies.
(1) Patched surfaces are inefficient representors of inner panel surfaces. The vastly different length scales associated with a multi-featured inner panel surface can be captured in a patched surface representation only through the use of large numbers of surface patches. A large inner panel surface, using current methods, may require over 1000 surface patches to achieve the desired shape. This is in contrast to a modest 10 or so patches required for an outer panel surface of similar size. While a large number of patches is not necessarily bad in itself, current procedures force the designer to define most patches individually, thus causing much of the difficulty associated with the current practices.
(2) Inner panels designed by current methods are difficult to modify. The designer must frequently respond to engineering changes to permit, for example, the mounting of a new or updated part on the inner panel. Using current methods, this entails the complete rebuilding of not only the relevant mounting surface but also much of the surrounding surface as well. To do this the designer must repeat the tedious and difficult process of defining patch boundaries, followed by the equally laborous process of adjusting patch parameters to achieve the desired surface shape as well as surface continuity.
(3) Rigorous continuity of normal derivatives across patch boundaries is not achieved using current methods. Along the boundary between two patches the normal derivatives associated with the component surfaces may suffer a jump discontinuity. While this may not detract from the visualization of the surface as seen on a cathode ray tube, such a discontinuity does complicate the generation of machine tool paths needed to cut the surface onto a die. Indeed, a significant portion of a tool path generator, is devoted to the generation of "guard surfaces" for the purpose of making a safe transition from one surface patch to another.
(4) The current data structure is not well matched to the character of a typical inner panel surface. The geometric building blocks in the current data structure for inner panels are parametric surface patches defined on rectangular grids in the parameter space. Individual features on the full surface can be modeled only by assembling large numbers of these patches - 25 or so for the simplest rectangular pocket shape. Moreover, any additional downstream processing (for example, NC path generation) must be done one patch at a time.